Methods for shifting loads

ABSTRACT

To produce repeated movements by means of alternately operating hydraulic jacks consisting of cylinder and piston units and having both supply sides and pressure sides, the pressure sides of the jacks are interconnected in such a manner as to form a closed pressure medium system while the supply sides are connected to a source of pressure medium in such a manner as to be alternately supplied with pressure medium for the movement contemplated, the pressure medium in the closed system being caused to flow between the pressure sides when the movement is performed. To shift a load by means of a group of simultaneously operating hydraulic jacks, individual jacks are temporarily relieved of load during the shifting movements thereof in that the support for said jacks is moved during shifting in a direction toward the load. To return a loaded or unloaded lifting element by means of alternately operating hydraulic jacks consisting of cylinder and piston units, the jacks having both supply sides and pressure sides as well as wedge-type jaws serving to bring about the engagement at the lifting operation, a counterweight attached to said lifting element is lifted in that the pressure sides are alternately supplied with pressure medium for the lifting operation and in that the jacks are provided with wedge-type jaws acting oppositely to the wedge-type jaws serving to realize the lifting operation. The return can also be brought about in that the jacks are provided with wedge-type jaws acting oppositely to the wedge-type jaws serving to realize the lifting operation, and in that the one sides of the jacks are interconnected in such a manner as to form a closed pressure medium system, while the other sides of the jacks are connected to a source of pressure medium in such a manner as to be alternately supplied with pressure medium for the movement of the lifting element, the pressure medium in the closed system being caused to flow via a counterpressure valve or like means between the two sides of the closed system when the movement is performed.

United States Patent 9] Ahlgren 1 1 METHOD FOR SHlFTlNG LOADS Nils Harald Ahlgren, Skyttevagen 22. 133 O0 Saltsjobaden, Sweden {22] Filed: Nov. 19, 1973 [21] Appl. No.: 417,141

[76] lnventor:

[52] US. Cl 254/1; 254/106 [51] Int. Cl B66f 1/00 [58] Field of Search 254/89 H. 105-107 [56] References Cited UNITED STATES PATENTS 3,522,931 3/1970 Heidenstam 254/105 3,556,480 1/1971 .lohansson 254/89 H 3,572,636 3/197] Johansson r 254/106 3,685 80l 8/1972 Ahlgren 254/105 Primary Examiner-Donald G. Kelly Assistant Examiner-Robert C. Watson [57] ABSTRACT June 17, 1975 pressure sides when the movement is performed. To shift a load by means of a group of simultaneously operating hydraulic jacks, individual jacks are temporarily relieved of load during the shifting movements thereofin that the support for said jacks is moved during shifting in a direction toward the load. To return a loaded or unloaded lifting element by means of alternately operating hydraulic jacks consisting of cylinder and piston units, the jacks having both supply sides and pressure sides as well as wedge-type jaws serving to bring about the engagement at the lifting operation. a counterweight attached to said lifting element is lifted in that the pressure sides are alternately supplied with pressure medium for the lifting operation and in that the jacks are provided with wedge-type jaws acting oppositely to the wedge-type jaws serving to realize the lifting operation The return can also be brought about in that the jacks are provided with wedge-type jaws acting oppositely to the wedge-type jaws serving to realize the lifting operation. and in that the one sides of the jacks are interconnected in such a manner as to form a closed pressure medium system, while the other sides of the jacks are connected to a source of pressure medium in such a manner as to be alternately supplied with pressure medium for the movement of the lifting element. the pressure medium in the closed system being caused to flow via a counterpressure valve or like means between the two sides of the closed system when the movement is performed.

6 Claims, 16 Drawing Figures PATENTEIJJUM 7 I975 I SHEET .Umik

PATENTEDJUN 17 1915 SHEET PATENTEDJUN 1 7 1975 SHEET METHODS FOR SHIFTING LOADS FIELD OF THE INVENTION This invention relates to methods and apparatuses for rapidly shifting loads.

More particularly, the invention relates to a method of producing repeated movement by means of alternately operating hydraulic jacks consisting of cylinder and piston units and having both supply sides and pressure sides. In this method the pressure sides of the jacks are interconnected in such a manner as to form a closed pressure medium system, while the supply sides of the jacks are connected to a source of pressure medium so that they are alternately supplied with pressure medium to bring about the movement contemplated, the pressure medium in said closed system being caused to flow between the pressure sides of the jacks when said movement is performed. The pistons of the jacks are thus returned during operation of the jacks, without necessitating supply of pressure medium for the return movement from the pressure medium source proper.

The invention also relates to a method of shifting a load by means of a group of simultaneously operating hydraulic jacks. This further method is characterised by temporarily relieving individual jacks of load during the shifting movements thereof in that the support for said jacks is moved during shifting in a direction toward the load. Therefore, a continuous raising or lowering movement ofa load need not be interrupted because an individual jack has to be relieved of load. After being relieved of load the jack can be loaded again in that its support is moved during shifting in a direction away from the load.

The apparatus for practising the method is a jointing device for rod-shaped lifting elements, said device comprising a frame having at its ends oppositely directed wedgetype jaw housings or like means which are adapted to engage the lifting element on either side of the joint thereof so that the load on the lifting element at said joint is transmitted to the frame.

Moreover, the invention relates to a method of returning a loaded or unloaded lifting element by means of alternately operating hydraulic jacks employed for the lifting of the element and consisting of cylinder and piston units, said jacks having supply sides and pressure sides as well as wedge-type jaws serving to bring about the engagement at the lifting operation. According to this further method, the return movement of the lifting element is produced by the lifting of a counterweight attached to said element, by alternately supplying the pressure sides with pressure medium for said lifting operation, and the jacks are provided with wedge-type jaws acting oppositely to the wedge jaws serving to realize the lifting operation. The use of the counterweight will eliminate the risk that the lifting element by its own weight will carry the piston in the jack along, thereby providing an uncontrolled rapid lowering movement.

Finally, the invention relates to a method of returning a loaded or unloaded lifting element by means of alternately operating hydraulic jacks employed for the lifting of said element and consisting of cylinder and piston units, said jacks having both supply sides and pressure sides as well as wedge-type jaws serving to bring about the engagement at the lifting operation. This method comprises providing the jacks with wedge-type jaws acting in a direction opposite to the wedge-type jaws serving to realize the lifting operation, and by interconnecting the one sides of the jacks in such a manner as to form a closed pressure medium system, while connecting the other sides of the jacks to a source of pressure medium in such a manner as to be alternately supplied with pressure medium for the movement of the lifting element, the pressure medium in the closed system being caused to flow via a counterpressure valve or like means between the two sides of the closed system at the movement of lifting element. The counterpressure valve employed in this method has fundamentally the same function as the counterweight of the immediately preceding method.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully described hereinbelow with reference to the accompanying drawings in which:

FIGS. 1-12 diagrammatically illustrate how a jack in a group of simultaneously operating hydraulic lifting jacks is temporarily relieved'of load and is then again loaded, and also show a specific coupling between the pressure sides of the jacks as well as a preferred embodiment of the jointing device for rod-shaped lifting elements;

FIGS. 13 and 14 illustrate a method of returning a lifting element;

FIGS. 15 and 16 illustrate another method of returning a lifting element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1 designates a jack assembly which is one of a group of identical jack assemblies, each of which co-operates with a lifting wire rope or other rod-shaped lifting element 2. The jack assembly I comprises two conventional superimposed cylinder and piston units 3 and 4, the pistons 5 of which are provided with wedge-type jaws 6 for engaging the lifting element 2 during a lifting operation. In the usual manner the jaws 6 slide on a conical surface, and alternately jaws of the upper and lower units 3 and 4 engage the lifting element 2, or one displaced relative to the lifting element. See, for example, FIGS. 1 and 2, as well as Ahlgren et al U.S. Pat. No. 3,685,801, issued Aug. 22, I972. The jack assembly 1 is placed on a cylinder and piston unit 7 which cooperates directly or via spacers 8 with a fixed support 9 through which the lifting element 2 passes.

The hydraulic cylinder and piston units 3 and 4 of the jack assembly 1 are adapted to provide a repeated movement by means of the alternately acting pistons 5 which have both first or supply sides and second or pressure sides with respect to the piston. The first or pressure sides of all of the jack assemblies 1 are interconnected via conduit means including a pipe I0. The second or supply sides are connected via pipes 11 and 12 to an external pressure source (not shown) so as to be alternately supplied with pressure medium for the movement contemplated. When pressure medium is supplied through one pipe I] or 12, respectively, pressure medium will thus be discharged through the other pipe 12 or 11, respectively. At the movement of the pistons 5 the pressure medium is caused to flow between the pressure sides through the pipe 10. It will be seen that when all of the pressure sides or first sides of the units 3 and 4 are interconnected through pipes 10, there exists a closed pressure medium system.

The pressure medium in the closed system is initially supplied though the pipe 13, FIGS. 1 and 2, which after supply has been realized is shut off by means of a shutoff valve 13a. By supplying the closed system formed by the pressure sides of the jacks with a quantity of pressure medium larger than that which corresponds to the volume of one of the pressure sides and the conduit means 10, the stroke length of the jacks 3 and 4 is reduced. For example, if additional fluid in excess of the volume of the first or pressure chamber is introduced to the closed system, the piston in the other cylinder of the system will be forced downward.

Normally, the closed system is supplied via the pipe 13 with a quantity of pressure medium larger than that which it shall contain during repeated movements. The excess pressure medium is then caused to leave the system through the overflow valve 14 when one ofthe supply sides is connected to the pressure medium source. When the desired quantity of pressure medium has left the closed system, the counterpressure in the overflow valve 14 is adjusted so that said valve will function as a safety valve.

It should be noted that the apparatus of FIGS. 3 12 also has the overflow valve 14, but that it is not shown for the sake of simplicity; and because when the apparatus is operating, the overflow valve functions as a safety valve.

As noted in FIG. I, the piston 5 in the cylinder and piston unit 4 has been moved to its upper extreme position by the action of pressure medium supplied through the pipe 12, the jaws 6 associated with said piston 5 having carried the lifting element 2 along. At the same time the piston 5 in the cylinder and piston unit 3 has been moved by the intermediary of the pressure medium in the closed system to its lower extreme position, the pressure medium at the supply side of the cylinder and piston unit 3 in turn having escaped through the pipe 11. In FIG. 2, the piston 5 of the cylinder and piston unit 3 is shown to have effected its lifting movement with the jaws 6, having clamped against the lifting element 2 while the piston 5 in the cylinder and piston unit 4 by the intermediary of the pressure medium in the closed system has been returned to its lower extreme position. In FIG. 3, a further lifting movement has been realized, which will appear from the position of the joint of the lifting element 2.

In FIG. 4, the jack assembly I has been relieved of load during the lifting movement thereof, in that the support structure for said jack assembly 1 during lifting simultaneously with a lifting operation or between two successive lifting operations has been moved in the direction opposite to the lifting direction. The other jack assemblies 1 (not shown) have on the other hand, retained their positions in relation to said support struc ture during the lifting movement, and as a consequence will wholly or partly take over the load from the thus relieved jack assembly 1. The movement of the support structure against the lifting direction has been performed by the piston 7a of the cylinder and piston unit 7 having been pushed inwardly in the cylinder which is shown in FIG. 4 to now co-operate with the fixed support 9 over spacers 16. The spacers 8 which previously co-operated with the piston 7a of the cylinder and piston unit 7, as shown in FIGS. 1 to 3, have been dispensed with.

In FIG. 5, the piston 7a of the cylinder and piston unit 7 during the continued lifting movement of the jack assembly 1 has been moved outwardly in the cylinder into engagement with the fixed support 9 which is now penetrated from the underside by the upper ends of rodshaped portions ofa jointing device 17 for the element The jointing device 17 comprises a frame having at its ends oppositely directed or oppositely acting wedge type jaw housings I8 and I9 which are adapted to engage the lifting element 2 on either side of the joint 15 which has the same cross-section as that of the lifting element 2 in order to pass through the jack assembly I so that the load in the lifting element 2 at the joint 15 may be transmitted to the frame. More particularly, the jointing device 17 comprises transverse members 20 and 21 to which the wedge-type jaw housings I8 and 19 are mounted, and also a number of longitudinal rodshaped elements 22 which interconnect the transverse members 20 and 21. The upper end portions 23 of elements 22 protrude above the upper transverse member 21. Because the cross-section of the joint 15 is the same as that of the lifting element 2, the joint 15 may pass through the jaws 6 ofjacks 3 and 4, and the jaw housing 18 and 19 of jointing device 17.

In FIG. 6, the end portions 23 are shown to be provided with supporting elements 24 which, as they engage the upper side of the fixed support 9, carry the jointing device 17. It will also be noted that the spacers 16 have been dispensed with, so that the jack assembly 1 rests only by means of the piston 7a of the cylinder and piston unit 7 on the fixed support 9.

In FIG. 7, there has again occurred a movement against the lifting direction, inasmuch as the piston 7a of the cylinder and piston unit 7 has been pushed inwardly in the cylinder. By this movement the supporting elements 24 have been brought into engagement with the support 9.

In FIG. 8, the wedge-type jaw housing 19 has been brought out of its engagement with the lifting element 2, so that the transverse member 21 has been moved slightly along the elements 22. As the lifting movement continues, load-relieved jack assembly 1 will again raise the lifting element 2 as will appear from the position of the joint 15 in FIG. 9; and the jointing device 17 retaining its position beneath the fixed support 9. When the joint 15 has passed the jack assembly 1 as shown in FIG. 9, the jack assembly can again be loaded because the support for said assembly during lifting has been moved in the lifting direction.

Thus, in FIG. 10, the piston 7a of the cylinder and piston unit 7 has been moved outwardly in the cylinder, thereby raising the jack assembly. Spacers 16 are then placed between the cylinder of the cylinder and piston unit 7 and the fixed support.

In FIG. 11, the piston 7a of the cylinder and piston unit 7 has been moved inwardly in the cylinder. and thereafter the spacers 8 have been placed between said piston'7a and the fixed support 9.

In FIG. 12, the jack assembly 1 has resumed its original position shown in FIG. I. The piston 7a of the cylinder and piston unit 7 has thus been moved outwardly in the cylinder so that said piston is supported by the intermediary of the spacers 8 against the fixed support 9.

When a loaded or unloaded lifting element is returned by means of alternately operating hydraulic jacks employed for the lifting of said element, each consisting of cylinder and piston units and having both supply sides and pressure sides as well as wedge-type jaws for the engagement of the lifting element during a lifting operation. there is the risk that a conventional lifting element which usually consists of a lifting wire rope may take the piston along, thus running away when the two parts of the lifting wire rope balance one another. This risk is eliminated by a method whose steps are illustrated in FIGS. 13 and 14. The return movement of the lifting element 30 is thus brought about by lifting a counterweight 31 fastened to the said element 30, and such lifting is realized by alternately supplying the pressure sides of the jacks 32 and 33 with pressure medium for said lifting operation. This also presupposes that the jacks 32 and 33 are provided with wedge-type jaws 36 and 37 serving to bring about said lifting movement and acting oppositely to the wedgetype jaws 34 and 35, said jaws 36 and 37 being alternately engaged with and disengaged from the element 30. In other words, jacks 32 and 33 may be operated in the usual manner as discussed above for lifting a load on lifting element 30; and when the lifting element is unloaded, for lifting the counterweight 31 by operating the jacks in the reverse manner and supplying pressure medium to the sides of the jacks which were previously the pressure sides from an external pressure source (not shown), and by connecting the sides of the jacks which were previously the supply sides in a closed pressure medium system. As noted above, the wedge-type jaws 34 and 35 engage the lifting element 30 during a lifting operation, and the wedge-type jaws 36 and 37 engage the lifting element 30 during a return operation; because of the oppositely directed forces within the jacks 32 and 33 depending on which side of each jack is connected to the external pressure source. The return run of the lifting element 30 co-operates with a suitable runway 38 about which it is curved and from which it thus extends down to the counterweight 31 suspended thereon.

The above-mentioned risk in returning a loaded or unloaded lifting element is eliminated also by means of a method whose steps are illustrated in FIGS. and 16. In this case too, the method is performed by alternately operating hydraulic jacks 42 and 43 consisting of cylinder and piston units, said jacks having both supply sides and pressure sides as well as wedge-type jaws 44 and 45 for the engagement at the lifting operation. According to this method, the jacks 42 and 43 are provided with wedge-type jaws 46 and 47 acting oppositely to the wedge-type jaws 44 and 45. The operation of jacks 42 and 43 is the same as the operation ofjacks 32 and 33, with wedge-type jaws 44 and 45 engaging the lifting element 40 during a lifting operation and wedgetype jaws 46 and 47 engaging the lifting element 40 during a return operation. As well, one side of each of the jacks 42 and 43 is interconnected to the other jack in a manner so as to form a closed pressure medium system 49, while the other sides of each of the jacks 42 and 43 are connected via the pipes 50 and 51 to a source of pressure medium in such a manner as to be alternately supplied with pressure medium for the movement of the lifting element 40, the pressure medium in the closed system 49 being caused to flow via a counterpressure valve or like throttling means 52 between the two sides of the closed system at said movement of the lifting element. The means 52 in the closed system 49 thus prevents the pistons in the jacks 42 and 43 from running away in their cylinders.

When the return movement of the lifting element 40 is performed by continued coaction with the wedgetype jaws 44 and 45 for the engagement at the lifting operation, the pressure sides of the jacks 42 and 43 are interconnected in a manner to form the closed pressure medium system 49. In the opposite case, when the return movement of the lifting element 40 is performed by coaction with the wedge-type jaws 46 and 47 acting oppositely to the jaws 44 and 45 serving to bring about lifting, the supply sides of the jacks 42 and 43 are interconnected so that they will form a closed pressure medium system. It should be observed that the return movement of the lifting element 40 occurs not only before the lifting of a load but that it may occur also before the lowering of a load. The lifting element 40 may suitably be a rod element which can take up both tension and compression.

The counterpressure valve 52 is equipped with an operating means 53 for its setting. The closed pressure medium system 49 includes a number of non-return valves which force the pressure medium flowing between the two sides of the system always to pass the counterpressure valve 52 against the action thereof. The counterpressure valve 52 therefore can be of the non-return valve type.

What I claim and desire by Letters Patent is:

1. A method of producing a repeated movement of gripping elements associated with piston elements of hydraulic jacks with respect to a lifting element passed therethrough by means of alternately operating pairs of hydraulic jacks consisting of cylinder and piston elements each having both a supply side and a pressure side with respect to the piston element, including the steps of supplying the pressure sides of the jacks with pressure medium and interconnecting the pressure sides of the jacks in such a manner as to form a closed pressure medium system;

connecting the supply sides of the jacks to an external pressure source for supplying pressure medium so that said supply sides are alternately supplied with pressure medium; the pressure medium in said closed system being caused to flow between the pressure sides of the jacks when movement of the gripping elements of said hydraulic jacks is caused by supply of pressure medium to one or the other of the pressure sides thereof.

2. A method as claimed in claim 1, wherein the closed system formed by the pressure sides of said jacks is supplied with a quantity of pressure medium larger than that which corresponds to the volume of one of the pressure sides and the interconnecting conduit means, so that the stroke length of the jacks is reduced.

3. A method as claimed in claim 1, wherein the closed system formed by the pressure sides of said jacks is first supplied with a quantity of pressure medium larger than that which said closed system shall contain during repeated movements, the excess pressure medium being caused to leave the closed system through an overflow valve when one or the other of the supply sides of the hydraulic jacks is first connected to said external pressure source.

4. A method of lifting and returning a lifting element by means of alternately operating pairs of hydraulic jacks employed for the lifting of said element, each said jack consisting of cylinder and piston elements; where the cylinders of each of said jacks have first and second sides with respect to the piston element and first wedge-type jaws associated with said piston element which engage said lifting element during the lifting operation;

said lifting operation being brought about by supplying the first sides of the jacks with pressure medium and interconnecting said first sides of said jacks so as to form a first closed pressure medium system; said second sides of said jacks being connected to an external source for supplying pressure medium; and alternately supplying said second sides with pressure medium; the pressure medium in said first closed pressure system being caused to flow between said first sides of the jacks when a lifting movement of said first wedge-type jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said second sides of said jacks;

said return movement of said lifting element occurring when a counterweight is attached to said lifting element and the piston element of each jack has second wedge-type jaws associated therewith which engage said lifting element during the return movement thereof, and being brought about by supplying the second sides of the jacks with pressure medium and interconnecting said second sides of said jacks so as to form a second closed pressure medium system; said first sides of said jacks being connected to an external source for supplying pressure medium; and

alternately supplying said first sides of said jacks with pressure medium; the pressure medium in said second closed pressure system being caused to flow between said second sides of the jacks when a return movement of said second wedge-type jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said first sides of said jacks.

5. A method of lifting and returning a lifting element by means of alternately operating pairs of hydraulic jacks employed for the lifting of said element, each said jack consisting of cylinder and piston elements; where the cylinders of each of said jacks have first and second sides with respect to the piston element and first 8 wedge-type jaws associated with said piston element which engage said lifting element during the lifting operation; said lifting operation being brought about by supplying the first sides of the jacks with pressure medium and interconnecting said first sides of said jacks so as to form a first closed pressure medium system; said second sides of said jacks being connected to an external source for supplying pressure medium; and alternately supplying said second sides with pressure medium; the pressure medium in said first closed pressure system being caused to flow between said first sides of the jacks when a lifting movement of said first wedgetype jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said second sides of said jacks;

each piston element of each of said jacks having second wedge-type jaws associated therewith and acting in a direction opposite to said first wedge-type jaws, so as to engage said lifting element during the return movement thereof; said return movement of said lifting element being brought about by supplying the second sides of the jacks with pressure medium and by interconnecting said second sides of said jacks in such a manner as to form a second closed pressure medium system, while connecting the first sides of the jacks to an external pressure source for supplying pressure medium in such a manner that said first sides are alternately supplied with pressure medium to cause movement of said piston elements and said second wedge-type jaws into engagement with said lifting element for return movement of the lifting element; the pressure medium in said second closed system being caused to flow via counterpressure valve means between the seconds sides of the jacks during a return movement of the lifting element. 6. A method as claimed in claim 5 where, when the lifting element is returned by coaction of said first wedge-type jaws therewith, said method includes the further step of interconnecting the second sides of said jacks in such a manner as to form a closed pressure medium system. 

1. A method of producing a repeated movement of gripping elements associated with piston elements of hydraulic jacks with respect to a lifting element passed therethrough by means of alternately operating pairs of hydraulic jacks consisting of cylinder and piston elements each having both a supply side and a pressure side with respect to the piston element, including the steps of supplying the pressure sides of the jacks with pressure medium and interconnecting the pressure sides of the jacks in such a manner as to form a closed pressure medium system; connecting the supply sides of the jacks to an external pressure source for supplying pressure medium so that said supply sides are alternately supplied with pressure medium; the pressure medium in said closed system being caused to flow between the pressure sides of the jacks when movement of the gripping elements of said hydraulic jacks is caused by supply of pressure medium to one or the other of the pressure sides thereof.
 2. A method as claimed in claim 1, wherein the closed system formed by the pressure sides of said jacks is supplied with a quantity of pressure medium larger than that which corresponds to the volume of one of the pressure sides and the interconnecting conduit means, so that the stroke length of the jacks is reduced.
 3. A method as claimed in claim 1, wherein the closed system formed by the pressure sides of said jacks is first supplied with a quantity of pressure medium larger than that which said closed system shall contain during repeated movements, the excess pressure medium being caused to leave the closed system through an overflow valve when one or the other of the supply sides of the hydraulic jacks is first connected to said external pressure source.
 4. A method of lifting and returning a lifting element by means of alternately operating pairs of hydraulic jacks employed for the lifting of said element, each said jack consisting of cylinder and piston elements; where the cylinders of each of said jacks have first and second sides with respect to the piston element and first wedge-type jaws associated with said piston element which engage said lifting element during the lifting operation; said lifting operation being brought about by supplying the first sides of the jacks with pressure medium and interconnecting said first sides of said jacks so as to form a first closed pressure medium system; said second sides of said jacks being connected to an external source for supplying pressure medium; and alternately supplying said second sides with pressure medium; the pressure medium in said first closed pressure system being caused to flow between said first sides of the jacks when a lifting movement of said first wedge-type jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said second sides of said jacks; said return movement of said lifting element occurring when a counterweight is attached to said lifting element and the piston elemeNt of each jack has second wedge-type jaws associated therewith which engage said lifting element during the return movement thereof, and being brought about by supplying the second sides of the jacks with pressure medium and interconnecting said second sides of said jacks so as to form a second closed pressure medium system; said first sides of said jacks being connected to an external source for supplying pressure medium; and alternately supplying said first sides of said jacks with pressure medium; the pressure medium in said second closed pressure system being caused to flow between said second sides of the jacks when a return movement of said second wedge-type jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said first sides of said jacks.
 5. A method of lifting and returning a lifting element by means of alternately operating pairs of hydraulic jacks employed for the lifting of said element, each said jack consisting of cylinder and piston elements; where the cylinders of each of said jacks have first and second sides with respect to the piston element and first wedge-type jaws associated with said piston element which engage said lifting element during the lifting operation; said lifting operation being brought about by supplying the first sides of the jacks with pressure medium and interconnecting said first sides of said jacks so as to form a first closed pressure medium system; said second sides of said jacks being connected to an external source for supplying pressure medium; and alternately supplying said second sides with pressure medium; the pressure medium in said first closed pressure system being caused to flow between said first sides of the jacks when a lifting movement of said first wedge-type jaws engaging said lifting element is caused by the supply of pressure medium to one or the other of said second sides of said jacks; each piston element of each of said jacks having second wedge-type jaws associated therewith and acting in a direction opposite to said first wedge-type jaws, so as to engage said lifting element during the return movement thereof; said return movement of said lifting element being brought about by supplying the second sides of the jacks with pressure medium and by interconnecting said second sides of said jacks in such a manner as to form a second closed pressure medium system, while connecting the first sides of the jacks to an external pressure source for supplying pressure medium in such a manner that said first sides are alternately supplied with pressure medium to cause movement of said piston elements and said second wedge-type jaws into engagement with said lifting element for return movement of the lifting element; the pressure medium in said second closed system being caused to flow via counterpressure valve means between the seconds sides of the jacks during a return movement of the lifting element.
 6. A method as claimed in claim 5 where, when the lifting element is returned by coaction of said first wedge-type jaws therewith, said method includes the further step of interconnecting the second sides of said jacks in such a manner as to form a closed pressure medium system. 